Microwave discriminator and demodulator/receiver using the same

ABSTRACT

A microwave frequency discriminator, i.e. an electronic device for directly transforming frequency modulation on a microwave carrier into a demodulated lower frequency signal. The discriminator is similar to a Travis discriminator and includes an inlet microstrip line, two resonant circuits (R 1 , R 2 ) constituted by dielectric resonators and coupled to said inlet microstrip line to receive the modulated microwave signal, two outlet microstrip lines coupling each resonator to a respective microwave detector circuit, said detector circuits including loads (r 1 , r 2 ) in series-opposition in the manner of a Travis discriminator. The microwave frequency discriminator can be used in a demodulator and/or receiver for frequency modulated microwaves.

This is a division of application Ser. No. 823,862, filed Jan. 29, 1986,now U.S. Pat. No. 4,694,260.

The present invention relates to a frequency discriminator, i.e. anelectronic device for transforming a frequency modulated signal into alow frequency demodulated signal, said discriminator being for use withmicrowaves. The invention also relates to devices making use of such adiscriminator.

BACKGROUND OF THE INVENTION

Generally speaking, a frequency discriminator is a device fortransforming frequency modulation into a low frequency demodulatedsignal and is usable in several applications, of which the most commoninclude Intermediate Frequency (IF) demodulation and Automatic FrequencyControl (AFC) of an oscillator, in which the discriminator is alsocaused to operate at the intermediate frequency.

At such "intermediate" frequencies which are generally several tens ofmegahertz (MHz) in a microwave transmitter or receiver, for example, itis conventional to use a "Travis discriminator" comprising twooscillating circuits tuned to two different frequencies F₁ and F₂situated on either side of the IF, together with two diode detectorcircuits followed by respective filter circuits each including acapacitor connected in parallel with a load resistor. By connecting thetwo load resistors in series opposition (push-pull), and by taking theoutput voltage from across the terminals of the total load thusobtained, a voltage is obtained having the conventionalamplitude-frequency characteristics A(f) shown in accompanying FIG. 1. Asignal S_(f) carried by a carrier wave F_(i) and frequency modulated isthus transformed into an amplitude modulated signal F_(a) by means of adiscriminator having a response curve A(f) as shown diagrammatically inFIG. 1.

Microwave frequency discriminators have been implemented for a very longtime, as can be seen from the book "Technique of Microwave Measurements"by C. G. Montgomery and published by McGraw-Hill Book Co., Inc., 1947,at pages 63 to 66. In those days the microwave circuits comprisedmagic-Ts, a metal microwave cavity, and waveguides having the propertyof reproducing an amplitude/frequency curve which is similar, atmicrowaves, to that of the Travis discriminator.

Such discriminators did not give rise to industrial implementationsbecause they are difficult to make, bulky, expensive, and highlysensitive to temperature differences. These drawbacks had the effect ofdissuading the person skilled in the art from using such circuits asdemodulators or as AFCs, for example, and generally speaking asmicrowave discriminators. Indeed, over the last thirty years or so,these drawbacks have established a marked prejudice against suchcircuits so that frequency discriminators have been implemented in thepath solely at IF.

Dielectric resonators for use with microwaves have been known sincebefore 1940 (see "General of Applied Physics", Volume 10, June 1939,pages 391 to 398). Such dielectric resonators have been used since thebeginning of the 60s in microwave applications: see in particular:

Proceedings IRE, Vol. 50, October 1962, pages 2081 to 2092;

IEEE transactions on Microwave Theory and Techniques, Vol. MTT-12,September 1964, pages 549 and 550; and

IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-13, March1965, page 256.

More recently still, microwave oscillators have appeared in which theoscillator circuit is constituted by a dielectric resonator.

However, heretofore such facts have not been able to overcome the verymarked prejudice of the person skilled in the art against using afrequency discriminator directly at microwave frequencies, and it hasthus not occurred to the person skilled in the art that the very smallsize of a dielectric resonator could be used to implement a microwavediscriminator which avoids the drawbacks of prior art microwavediscriminators. It must also be added that dielectric resonators sufferfrom considerable temperature drift, as witnessed by the above-mentionedliterature, and that the person skilled in the art is used to afrequency discriminator requiring an amplitude/frequency response curvewhich is particularly stable as a function of temperature.

Preferred embodiments of the present invention provide a microwavefrequency discriminator which avoids the abovementioned drawbacks ofprior art devices of this type.

SUMMARY OF THE INVENTION

The present invention provides a microwave frequency discriminator,comprising:

a microwave inlet circuit for a microwave to be discriminated;

a pair of microwave dielectric resonators having resonant frequencies F₁and F₂ situated on either side of the frequency F₀ of the microwave tobe discriminated;

a first microwave coupling device extending between said inlet circuitand each of said resonators;

a pair of detector circuits;

respective microwave coupling devices extending between each of saidresonators and corresponding respective ones of said pair of detectorcircuits, said detector circuits having output terminals which areconnected in series-opposition in the manner of a Travis discriminator;and

a discriminated microwave outlet taken from said output terminals ofsaid pair of detector circuits.

More particularly, the present invention relates to a discriminator asdefined above and comprising:

a protective enclosure;

a common substrate located in said enclosure;

said pair of dielectric resonators being placed on said substrate in theimmediate proximity of a first microstrip also placed on said substrateand constituting said first microwave coupling device;

two second microstrips placed on said substrate in the immediateproximity of respective ones of said resonators and constituting saidsecond microwave coupling devices; and

said pair of microwave detectors loading respective ones of said secondmicrostrips, each of said detectors being associated with a respectivedetector resistance.

The invention also provides an automatic frequency control (AFC) devicefor a microwave oscillator, said oscillator having an output fordelivering an oscillation and a control input for receiving afrequency-controlling input signal, said AFC device comprising:

a microwave coupling device placed at the output from said oscillator;and

a microwave frequency discriminator as defined above, said discriminatorhaving a center frequency F_(O) equal to the nominal frequency of saidoscillator, the inlet to said discriminator being fed from said couplingdevice, and the outlet from said discriminator being connected todeliver a frequency-controlling signal to said oscillator control input.

In a particularly advantageous embodiment, such an AFC device has amicrowave frequency discriminator in which the pair of resonators aresuch that their resonant frequencies vary in opposite directions withchanges in temperature. This avoids the need for any ancillarytemperature correcting means.

The invention also provides a demodulator for frequency modulatedmicrowaves, said demodulator comprising in succession:

a microwave amplitude stabilizer; and

a microwave discriminator as defined above and including dielectricresonators.

In an advantageous embodiment of such a demodulator, said amplitudestabilizer comprises one or more series-connected units, eachcomprising:

a synchronized oscillator and a microwave circulator having at leastthree ports, namely:

a first port for receiving microwaves to be demodulated;

a second port in the direction of circulation, said second port beingconnected to output said received microwaves to said synchronizedmicrowave oscillator, and also to receive oscillations output from saidsynchronized oscillator; and

a third port in the direction of circulation for outputtingamplitude-stabilized microwaves.

The invention also provides an amplitude stabilizer for microwavefrequencies as defined above.

The invention additionally provides a receiver for frequency modulatedmicrowaves, said receiver comprising in succession:

a filter device for filtering a carrier wave at a frequency which it isdesired to receive; and

a microwave demodulator as defined above.

Finally, the invention provides a multichannel receiver for frequencymodulated microwaves, said receiver comprising in succession:

a filter for selecting a carrier wave at a desired frequency;

a microwave demodulator as defined above;

a device for generating a direct voltage equal to the direct voltagewhich is superposed, at the output from said demodulator, on thedemodulated alternating voltage representative of the modulation carriedby said carrier wave; and

means for subtracting said generated direct voltage from the combineddirect and alternating voltages available at said demodulator output.

Advantageously such a multichannel receiver is fitted with means forautomatically selecting said desired frequency from said generateddirect voltage.

In a particular embodiment such a multichannel receiver is fitted withmeans for automatically controlling said filter as a function of saidgenerated direct voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a voltage/frequency response curve of a Travis frequencydiscriminator;

FIG. 2 is a circuit diagram of a microwave discriminator in accordancewith the invention;

FIG. 3 is a partially cutaway perspective view of a practicalimplementation of the FIG. 2 discriminator;

FIG. 4 shows the deformation of the response curve of the FIG. 3discriminator as a function of temperature;

FIG. 5 is a block diagram of an automatic frequency control device inaccordance with the invention for a microwave oscillator;

FIG. 6 is a diagram of a specific embodiment of the FIG. 5 device;

FIG. 7 is a diagram of a demodulator in accordance with the inventionfor a frequency modulated microwave;

FIG. 8 is a block diagram of a receiver in accordance with the inventionfor a frequency modulated microwave;

FIG. 9 is a diagram for explaining the possibility of multi-channelreception using a discriminator in accordance with the invention; and

FIG. 10 is a block diagram of a multi-channel receiver in accordancewith the invention for frequency modulated microwaves.

MORE DETAILED DESCRIPTION

FIG. 1, which has already been described above, shows a characteristicvoltage(A)/frequency(f) response of a Travis discriminator. The usefulportion of such a characteristic is the linear portion L lying betweenfrequencies F₁ and F₂ to which the two resonant circuits used are tuned.Such a linear portion is conventionally characterized by its qualitiesof linearity and slope, which, as is well known to the person skilled inthe art, are conflicting characteristics.

FIG. 2 is an electric circuit diagram of a microwave discriminator inaccordance with the invention and is constituted by transposing tomicrowave frequencies a Travis discriminator such as is commonly used atvery much lower frequencies. The transposition applies in particular tothe use of dielectric resonators and of a microstrip line.

In FIG. 2, reference 1 designates the inlet to the discriminator of afrequency modulated microwave at Frequency F₀. For example the wave mayhave carrier frequency of 7 GHz which is frequently modulated using amodulation channel having a total bandwidth of 50 MHz. As can be seen inthe drawing, this signal is received in a discriminator in accordancewith the invention on a microstrip inlet line 2 whose length isapproximately equal to one-half of the wavelength λ₀ of the inlet signalat frequency F₀, said strip having a characteristic impedance Z₀ of 50ohms, by way of typical example. The other end of the microstrip 2 isterminated by a terminating resistance 3 whose value R₀ is equal to 50ohms and whose other end is connected to ground. In a variant, the inlet1 could be coupled to two microstrip lines, each of them beingterminated on a terminating resistance and with a power dividerdistributing power between the two input lines.

In accordance with the invention, the line 2 is coupled on either sideto two dielectric resonators R₁ and R₂, e.g. made of zirconium titanate,with each of the resonators having the same coupling coefficient η whichdepends on the small gap e (a few tenths of a millimeter) left betweenthe line 2 at each of the resonators R₁ and R₂.

Each of the resonators R₁ and R₂ is tuned to a corresponding frequencyF₁ (which is less than the frequency F₀) or F₂ (which is greater thanthe frequency F₀). For example, for a carrier frequency F₀ of 7 GHz asmentioned above, the frequency F₁ is equal to 6 GHz and the frequency F₂is equal to 8 GHz, thereby giving a discrimination band (F₂ -F₁) ofapproximately 2 GHz.

As can be seen in the drawing, each dielectric resonator R₁, R₂ is alsocoupled to a respective further microstrip line 4 or 5, said lines beingidentical to the inlet line 2 and each being terminated, as shown, atone of its two ends by a corresponding terminating resistance 6 or 7whose value R₀ is equal to the characteristic impedance Z₀ of the line,i.e. 50 ohms in the present example.

In the example shown, the gap e left between each resonator R₁, R₂ andits associated second line 4, 5 is equal to the gap left between each ofthe resonators and the inlet line 2. Naturally, the gaps could bedifferent if so required.

The other ends of the lines 4 and 5 are terminated on respectivemicrowave detection circuits 8 and 9, each comprising a microwavedetection diode D₁, D₂, connected between the corresponding end andground and having a corresponding parallel stray capacitance Cd₁, Cd₂.In addition, respective load resistances r₁, r₂ having a value of a fewkilohms for example are also connected between said other ends of thelines 4 and 5 and ground. Detected voltages E₁ and E₂ thus appear acrossthe terminals of the load resistances r₁ and r₂, and if the outputvoltage S from the discriminator is taken, as shown, from across saidother ends of the lines 4 and 5, these detected voltages are in seriesopposition.

The operation of the FIG. 2 discriminator is identical, aftertransposition to microwave frequencies, to the operation of a Travisdiscriminator, with the resonators R₁ and R₂ replacing the conventionalinductance and capacitance resonators, thereby giving the FIG. 2discriminator the same response curve as that shown in FIG. 1 with thelower frequency F₁ being equal to 6 GHz, with the upper frequency F₂being equal to 8 GHz and with a middle frequency F_(i) thus being equalto the frequency F₀, i.e. to 7 GHz in the present example.

FIG. 3 is a perspective view of a practical implementation of the FIG. 2discriminator.

In FIG. 3, reference 10 designates a metal rectangular enclosure forreceiving the discriminator in accordance with the invention and forbeing closed, as indicated by arrow f by a metal cover 11. The resultingbox 11 constitutes ground for the device and is quite small at the 7 GHzfrequency under consideration, e.g. it may be 25×25×20 millimeters.

The inlet 1 of FIG. 2 is constituted by a coaxial base 12 which is fixedto the box 11 by screws 13. The dielectric resonators R₁ and R₂ and themicrostrip lines 5, 2, and 4 are glued to a common substrate 14 made ofinsulating material, e.g. alumina.

Advantageously, the dielectric resonators R₁ an R₂ are constituted byrespective materials which, while both being zirconium titanate, forexample, have respective thermal expansion coefficients which are equaland opposite to each other. This effect can be obtained by a chemicalprocess which is widely known at the present time. For example, theresonator R₁ may expand with increasing temperature such that itsresonant frequency F₁ decreases, while the resonator R₂ may shrink bythe same amount with increasing temperature such that its resonantfrequency F₂ increases by the same amount.

In conventional manner for dielectric resonators, metal plungers 15 and16 mounted on the box cover 11 are positioned opposite respective onesof the resonators R₁ and R₂ and are capable of being moved towards oraway from the resonators by means of associated knurled knobs 17 and 18.This serves to provide fine adjustment of the resonant frequencies F₁and F₂ of each of the resonators R₁ and R₂.

Advantageously, each of the plungers 15 and 16 may be of the"mecanothermal" or temperature compensation type, for example asdescribed in U.S. Pat. No. 3,528,042. In order to do this, the rod (19or 20 as the case may be) of each of the plungers 15 and 16 is made of amaterial such as Invar or such as a metal-plated inert quartz which isinert as a function of temperature, i.e. which has a very small orcompletely negligible temperature expansion coefficient. Under suchconditions, it appears that the metal tubular enclosure 21 of eachplunger increases in length with increasing temperature while itsassociated rod (19, 20) does not increase in length, such that eachplunger (15, 16) moves away from the associated dielectric resonator(R₁, R₂) thereby tending to increase the resonant frequency by an amountwhich is selected to cancel the reduction in resonant frequency whichcould otherwise be expected from the increase in temperature.

This applies so long as both resonators (R₁, R₂) have identicaltemperature properties. In the particular case mentioned above where oneof the resonators, e.g. R₁, is made of a material such that its resonantfrequency decreases with increasing temperature while the otherresonator, e.g. R₂, is made of a material such that its resonantfrequency increases with increasing temperature, it is necessary for thesecond plunger, i.e. 16 in this example, associated with the secondresonator R₂ should be made so that it moves closer to the resonatorwith increasing temperature, i.e. in the present example it could becarried by a rod 20 made of ordinary metal and contained in tubularenclosure 21 made of a thermally inert material.

FIG. 3 also shows detection diodes D₁, and D₂ which are connected asshown in FIG. 2, together with the terminating resistances 3, 6 and 7.As can be seen in the drawing, the diodes D₁, and D₂ are connected totheir respective load resistances r₁ and r₂ via choke coils 22 and 23and respective feedthroughs 24 and 25 which are insulated from the metalenclosure 10. As can be seen in the drawing, the other end of each ofthe load resistances r₁ and r₂ is soldered to a respective grounding tagsuch as 26.

In an advantageous embodiment of the FIG. 3 discriminator, thedielectric resonators R₁ and R₂ are made of materials such that theirresonant frequencies vary in opposite directions as a function oftemperature. FIG. 4 is used to explain the advantageous effect whichresults from this special disposition.

In FIG. 4, the characteristic L obtained at ambient temperature (e.g.20° C.) is drawn in solid lines, while the characteristic L' obtained ata temperature higher than ambient (e.g. 70° C.) is drawn in dashedlines. In the characteristic L' the resonant frequency F₂ of theresonator R₂ has been increased up to F'₂ and the resonant frequency F₁of the resonator R₁ has been reduced by the same amount to F'₁. Thelinear portions situated between the two "humps" of each of thecharacteristics L and L' are shown by respective straight lines 1 and1'.

As can be seen in FIG. 4, so long as the "humps" are moved symmetricallyby increasing temperature, the center frequency F₀ remains unchanged.Only the slopes of the linear portions L and L' are different, and insome applications such as automatic frequency controlled (AFC) this isnot of any great significance, as is explained below.

It may be observed that implementing a discriminator in accordance withthe invention and as shown in FIG. 3 gives rise to a device which isvery small. Under such conditions it is very easy to obtain temperaturestabilization by placing the device in a thermostatically controlledenclosure, which is maintained at a constant temperature, e.g. 75° C.The resulting enclosure or oven may be very small and thus easy toinstall, and also consuming very little energy. The possibility ofstabilization by means of a thermostatically controlled enclosure ispractically inconceivable with bulky devices using metal cavities,magic-Ts, and waveguides as taught in the prior art, yet it becomes verypractical with a small-sized device in accordance with the invention.

A microwave discriminator in accordance with the invention is suitablefor use in several novel implementations which are particularlyadvantageous. By virtue of the very high Q factor inherent to dielectricresonators, its slope is considerably higher than that of thediscriminators which are currently used at lower frequencies. Forexample the FIG. 3 embodiment gives a slope of 0.25 volts/MHz forfrequencies F₁ and F₂ which are 20 MHz apart.

These high slope figures thus make it possible to provide automaticfrequency control for a microwave oscillator in a simple manner sincethere is no need for frequency changing Such an oscillator may, forexample, be a high spectral purity oscillator such as a dielectricresonator oscillator or DRO.

Further, the possibility of obtaining a very large difference betweenthe extreme discrimination frequencies F₂ and F₁ in a discriminator inaccordance with the invention, e.g. 2 GHz, makes it possible to obtainan automatic frequency controlled device which always convergesregardless of temperature variations. It is thus possible to eliminatethe searching devices which have previously been necessary in AFCdevices, which searching devices are particularly complex and expensive.FIG. 5 is block diagram of an AFC for a microwave oscillator inaccordance with the invention.

In this figure, reference 27 designates an oscillator, e.g. a DRO,designed to oscillate at a frequency F₀ of 7 GHz, for example. Inconventional manner, this oscillator comprises an oscillating circuit 28whose frequency is controllable by an externally applied direct voltagewhich is applied at 29 to a controlled circuit 30, e.g. a circuitincluding varactor diodes.

The output from the oscillator 27 is connected to a microwave coupler 31at a few dBm for extracting a small amount of power at 32, e.g. about 1milliwatt. This power is supplied to the input of a discriminator 33having dielectric resonators with a center frequency F₀ such as thediscriminator shown in FIG. 3 which is equipped with resonators havingequal and opposite temperature variations. If the frequency of the waveapplied at 32 to the discriminator 33 moves away from the frequency F₀,the discriminator 33 provides an error voltage at 34 which is positiveor negative depending on the sign of the error, and this voltage isapplied via an analog level changing amplifier 35 to the control input29 of the varactor device 30, thereby returning the frequency of theoscillator 27 to its nominal value F₀. A signal at a constant frequencyF₀ is thus obtained at the output 36 from the circuit as a whole.

FIG. 6 illustrates an example of a practical embodiment of the circuitshown in FIG. 5.

As can be seen in the drawing, both the oscillator 27 (surrounded by anellipse in the drawing) and the discriminator 33 (also surrounded by anellipse in the drawing) are placed on a common substrate 40 which ismade of insulating alumina, for example. For maximum clarity, thereferences which correspond to items that appear in FIGS. 2 and 5 havebeen repeated unchanged in FIG. 6 and the corresponding items aretherefore not described a second time. The discriminator 33 used in thiscase is thus identical to the discriminator shown in FIG. 2, except forthe low frequency Choke coils l₁ and s₂ which were added in FIG. 2between the diodes D₁, and D₂ and the associated resistances r₁ and r₂.

The coupler 31 is constituted by a microstrip line which is terminatedby a matching resistance 41 which is placed, as shown, close to theoutput microstrip line 42 from the microwave oscillator 27, with thedegree of coupling depending on the power level of the output from theoscillator 27.

The output 34 from the discriminator 33 is taken in this case across theterminals of a resistance 43 which is star connected with theresistances r₁ and r₂. This is a slightly different arrangement to thatdescribed with reference to FIG. 2, but the result is electrically thesame.

The output from the low frequency amplifier 35 (e.g. an operationalamplifier) is connected at 29 to the control circuit 30 for controllingthe frequency of the oscillator 27 as a function of a voltage. Inconventional manner, this circuit comprises two varactor diodes D₃ andD₄ which are interconnected by a microstrip line 44 which is itselfcoupled to the microstrip "coupling" line 45 of the oscillator 27. In amanner which is entirely conventional in microwave circuits, the inlet29 is connected to the microstrip line 44 by means of a microwave andlow frequency decoupling circuit comprising a low frequency choke 46, amicrowave quarter wavelength trap 47, and a high impedance quarterwavelength line 48. The two varactor diodes D₃ and D₄ are used inconventional manner for increasing the linearity of the slope of theoscillator 27.

As shown in the drawing, the line 45 is terminated at one of its ends ona matching resistance 49 via a decoupling capacitance 50 having a valueof a few picofarads, for example the DC bias voltage for the base 55 ofthe transistor 56 in the oscillator 27 is also applied to the same endof the line 45 from a feed point 51 and via an additional decouplingcircuit likewise including a low frequency choke 52, a quarter wave trap53, and a quarter wave line 54.

The microstrip line 45 at base 55 is coupled, in a conventional manner,to a dielectric resonator R₀ whose resonant frequency is equal to thefrequency F₀ of the oscillator 27, i.e. to 7 GHz in the present example.

The transistor 56 (or any other suitable active electronic device, suchas a field effect transistor - FET) has its collector connected toground via fixing screws 57 and a clamp 58, while its emitter 59 isconnected firstly to a DC bias voltage applied at 60 via a quarterwavelength line 61, a trap 62 and a choke 63, and also via an additionaldecoupling capacitance 64 to the output microstrip line 42, which is inturn connected at its other end to the output connector 36 of theoscillator.

In accordance with another aspect of the invention, a discriminator inaccordance with the invention may be used in association with amicrowave level limiter to directly provide a microwave demodulator forfrequency modulated microwaves. The limiter may either be a conventionalmicrowave level limiter or else it may be a completely novel microwavelevel limiter as described below. Demodulation is provided much moresimply when performed directly than when performed at an intermediatefrequency as is the conventional case, and such direct demodulationmakes it possible, advantageously, to avoid many of the well-knownproblems of non-linearity which are introduced at the intermediatefrequency by the "limiting" diodes commonly used. Further, the microwavelimiter, and in particular the novel type of limiter which is describedbelow, may also be used to provide automatic gain control (AGC), therebyproviding additional simplification.

Reference is now made to FIG. 7 which is a diagram of a microwavedemodulator in accordance with the invention for demodulating afrequency modulated microwave.

The microwave having a carrier frequency F₀ of 7 GHz, for example, whichis frequency modulated over a bandwidth of 50 MHz, for example, isapplied to the demodulator in accordance with the invention at 70. Thiswave is applied to the inlet 71 of a first stage 72 of a novel type ofmicrowave amplitude stabilizer which also acts as an AGC device.

The stabilizer 72 comprises a three channel microwave circulator 73,e.g. a ferrite circulator. The signal is applied at 71 to the firstinlet channel. It leaves via the following outlet 173 in the directionof circulation and is applied by a line 74 to the frequency controlinput of a conventional synchronized oscillator 75 which thereforeretransmits a wave at the frequency F₀ =7 GHz over the line 74 towardsthe circulator 73. The retransmitted wave is of absolutely constantpower P_(s) which is a function only of the oscillator 75. This wave ofdefined frequency F₀ and power P_(s) passes through the circulator 73 inthe direction indicated by the arrow, and leaves via the second outlet76 to follow a line 77 either directly to the discriminator 78, orpreferably to a second amplitude stabilizer 79 which is similar to thestabilizer 72 but which has a higher output power in order to increasethe dynamic range over which stabilization and AGC can be performed.More than two stabilizing stages such as 72 and 79 are indicated in thefigure, and the last stage which defines the input power applied to thediscriminator 78 is shown in dashed lines 80.

A circuit 72 comprising a synchronized oscillator 75 and a circulator 73thus serves, in accordance with the invention, to provide a microwaveamplitude stabilizer, which stabilizer has the additional advantage ofoperating as an automatic gain control circuit. It should be observedthat such a circuit 72 comprising a circulator 73 and an oscillator 75is known per se, i.e. as a circuit for synchronizing a synchronizemicrowave oscillator. The invention thus consists in using this knowncircuit as an amplitude stabilizer, and consequently as an AGC, atmicrowave frequencies.

The compressed signal at the output from the last stabilizer stage 80 isthus applied at 81 to the input of the dielectric resonatordiscriminator 78, which is a discriminator such as described above withreferences to FIGS. 2 and 3, for example. The output S for thedemodulated wave is applied, as shown to an output amplifier, with thesaid low frequency wave which is directly demodulated without passingvia an intermediate frequency being made available at 83.

FIG. 8 is a diagram of a single channel microwave receiver in accordancewith the invention and operating, for example, at 7 GHz+25 MHz.

As can be seen in the drawing it comprises, in order:

a microwave inlet 85 for receiving microwaves from an antenna, forexample;

a channel filter 86 tuned to the channel to be received;

a microwave demodulator 87, identical to the FIG. 7 demodulator andtherefore not described in greater detail; and

a low frequency output amplifier 82.

Reference is now made to FIG. 9 which is used to explain a last aspectof the invention enabling a demodulator in accordance with the inventionto provide a simple implementation of a microwave multichannel receiver.

FIG. 9 shows the voltage(V)/frequency(f) response characteristic 90 of amicrowave discriminator in accordance with the invention centered on afrequency F₀ =7 GHz for example and having a useful bandwidth L of 2GHz, for example.

If such a discriminator is to be used in a demodulator in a multichannelreceiver, it can be seen that it is possible to demodulate a very largenumber of channels. In the present example, supposing each channel has afrequency bandwidth of 50 MHz, the discriminator with its usablefrequency bandwidth L equal to 2 GHz is capable of demodulating 40successive adjacent channels. In FIG. 9, the successive channels are:channel 1, channel 2, channel 3, . . . , channel N-1, channel N, andthey correspond to successive carrier frequencies F₁, F₂, F₃, . . . ,F_(N-1),, F_(N)

Except for the frequency F₀, the output of the demodulator will provide,as can be seen in FIG. 9, both a DC voltage V₁, V₂, V₃, . . . , V_(N-1),V_(N) for each channel on which an AC voltage v₁, v₂, v₃, . . . ,v_(N-1), v_(N) is superposed as demodulated from the correspondingchannel 1, 2, 3, . . . , N-1, N.

In accordance with the invention, a corresponding DC voltage V₁, V₂, . .. , V_(N) is generated elsewhere as a function of the frequency F1, F2,. . . , FN of the channel 1, 2, . . . , N under consideration. This DCvoltage, e.g. V₁, is then subtracted from the corresponding demodulatedvoltage V₁ +v₁ in order to obtain the required low frequency voltage v₁.In an advantageous implementation of the invention the DC voltage suchas V₁ is also used to provide automatic selection of the frequency, e.g.F₁, of the channel to be received.

FIG. 10 is a diagram of a multichannel receiver operating as describedabove.

This figure uses the same inlet line 85 for the channel signal from thereceiver antenna. The signal passes through a mechanically tunablechannel filter 91 and is then applied to a microwave demodulator 87which is identical to that described above with reference to FIG. 7. Thedemodulator provides a demodulated voltage at 92 which is equal, as hasbeen explained with reference to FIG. 9, to the sum of a DC voltage V₁and an AC voltage v₁ corresponding to the received channel which is atthe frequency F₁ in the present example.

Further, a digital signal constituted by a byte representative of thefrequency F₁ to be received is applied in parallel to the eight inputlines 93 of a digital to analog converter 94 which converts the 8 bitsof the byte into a DC voltage of amplitude equal to V₁ on its output 95.

The voltage V₁ at 95 is applied firstly via a line 96 to the negativeterminal of an analog summing circuit 97 whose positive terminalreceives the demodulated voltage V₁ +v₁ via the line 92 from thedemodulator 87. The output 98 of the subtractor constituted by thesumming circuit 97 thus provides a signal constituted by the lowfrequency AC component v₁ of the demodulated voltage. The DC voltage V₁available at 95 is also applied at 103 via a power level shiftingamplifier 99 to a DC stepper motor 100 whose shaft is mechanicallyconnected by a rod 101 to the mechanical inlet member for controllingthe adjustment of the filter 91. The items 100 to 103 are mechanicallyand electrically adjusted so that when the DC voltage V₁ is applied at103, the motor 100 turns through an angle suitable for setting the rod101 so that the filter 91 selects the frequency F₁.

We claim:
 1. A demodulator for frequency modulated microwaves, saiddemodulator comprising, in succession, a microwave amplitude stabilizerand a microwave discriminator, said microwave discriminator comprising:amicrowave inlet circuit for a microwave to be discriminated; a pair ofmicrowave dielectric resonators having resonant frequencies F₁ and F₂situated on either side of the frequency F₀ of the microwave to bediscriminated; a first microwave coupling device extending between saidinlet circuit and each of said resonators; a pair of detector circuits;respective microwave coupling devices extending between each of saidresonators and a corresponding respective one of said pair of detectorcircuits, said detector circuits having output terminals which areconnected in series-opposition in the manner of a Travis discriminator;and a discriminated outlet being taken from said output terminals ofsaid pair of detector circuits.
 2. A demodulator according to claim 1wherein said amplitude stabilizer comprises at least one unit, each unitcomprising a synchronized oscillator and a microwave circulator havingat least three ports, said at least three ports comprising:a first portfor receiving microwaves to be demodulated; a second port in thedirection of circulation, said second port being connected to outputsaid received microwaves to said synchronized oscillator, and also toreceive oscillations output from said synchronized oscillator; and athird port in the direction of circulation for outputting amplitudestabilized microwaves.
 3. A demodulator according to claim 1, whereinsaid resonators are made of respective materials such that the resonantfrequencies of said resonators tend to vary in opposite directions withchanging temperature.
 4. A receiver for frequency modulated microwaves,said receiver comprising, in succession, a filter device for filtering acarrier wave at a frequency which it is desired to receive, and amicrowave demodulator, said microwave demodulator comprising, insuccession, a microwave amplitude stabilizer and a microwavediscriminator, said microwave discriminator comprising:a microwave inletcircuit for a microwave to be discriminated; a pair of microwavedielectric resonators having resonant frequencies F₁ and F₂ situated oneither side of the frequency F₀ of the microwave to be discriminated; afirst microwave coupling device extending between said inlet circuit andeach of said resonators; a pair of detector circuits; respectivemicrowave coupling devices extending between each of said resonators anda corresponding respective one of said pair of detector circuits, saiddetector circuits having output terminals which are connected inseries-opposition in the manner of a Travis discriminator; and adiscriminated outlet being taken from said output terminals of said pairof detector circuits.
 5. A multichannel receiver for frequency modulatedmicrowaves, said receiver comprising, in succession, a filter forselecting a carrier wave at a desired frequency, a microwavedemodulator, a device for generating a direct voltage which is equal tothe direct voltage which is superposed, at the output from saiddemodulator, on the demodulated alternating voltage representative ofthe modulation carried by said carrier wave, and means for subtractingsaid generated direct voltage from the combined direct and alternatingvoltages available at said demodulator output, said microwavediscriminator comprising:a microwave inlet circuit for a microwave to bediscriminated; a pair of microwave dielectric resonators having resonantfrequencies F₁ and F₂ situated on either side of the frequency F₀ of themicrowave to be discriminated; a first microwave coupling deviceextending between said inlet circuit and each of said resonators; a pairof detector circuits; respective microwave coupling devices extendingbetween each of said resonators and a corresponding respective one ofsaid pair of detector circuits, said detector circuits having outputterminals which are connected in series-opposition in the manner of aTravis discriminator; and a discriminated outlet being taken from saidoutput terminals of said pair of detector circuits.
 6. A multichannelreceiver according to claim 5, fitted with means for automaticallyselecting said desired frequency from said generated direct voltage. 7.A multichannel receiver according to claim 6, fitted with means forautomatically controlling said filter as a function of said generateddirect voltage.